1. Field of the Invention
The present invention relates to a control device for a flash device used for a camera of the kind which automatically adjusts the amount of light emission by the flash device in such a way as to give an apposite exposure.
2. Description of Related Art
Flash-device control systems included in camera systems for performing exposure actions by emitting light toward an object of shooting and automatically adjusting the amount of light emission for an apposite exposure have variously been developed. These known flash-device control systems generally employ a so-called TTL light control method by which an apposite amount of light emission is obtained by measuring light reflected by a film surface after arrival of light at the film surface at the time of exposure.
Further, some of these known flash-device control systems have been developed to solve a problem which is peculiar to a focal plane shutter and lies in that a flash synchronizing time is limited as the focal plane shutter makes a slit exposure for a high shutter speed. According to this solution, preliminary light emission is made before exposure and the amount of emission of main light emission is computed on the basis of a measured value of light obtained at the time of the preliminary light emission and another measured value of light obtained immediately before the preliminary light emission.
In the system which makes the preliminary light emission before exposure and computes the amount of emission of the main light emission on the basis of a measured light value obtained at the time of the preliminary light emission and a measured light value obtained immediately before the preliminary light emission, measurement of light is performed by a light measuring sensor which is not located on the optical axis. Therefore, in order to obtain a correct measured light value, it is necessary to transmit information on the full-open aperture value of a photo-taking lens to the camera. However, in a case where the full-open aperture value of the photo-taking lens is unknown, such as a case where a bellows or an intermediate ring is interposed in between the photo-taking lens and the camera, it is hardly possible to obtain a correct measured light value. The operation of the system then tends to result in an underexposure or an overexposure.